The present invention relates to an electron gun for a cathode ray tube and a method of producing the electron gun.
Cathode ray tubes for television receivers and computer displays have been recently required to display an image with higher definition.
Along with this requirement, there is a trend toward the reduction in diameter of the electron beam aperture (hereinafter, referred to as xe2x80x9cbeam aperturexe2x80x9d) of each grid electrode of the electron gun used for cathode ray tubes.
For an electron gun used for displays, the diameter of the beam aperture of the first grid electrode closest to the cathode of the electron gun has been shifted yearly, for example, from xcfx860.43 mm to about xcfx860.32 mm, and further to about xcfx860.30 mm.
To drive such an electron gun in which the diameter of the beam aperture is reduced at the same voltage as a conventional voltage, it is required to reduce the gap between the cathode and the first grid electrode.
To realize a reduction in gap between the cathode and first grid electrode, it is required to make the thickness of the metal plate constituting the first grid electrode thin.
In actuality, along with the above-described shift of the diameter of the beam aperture, the thickness of a portion, of the first grid electrode around the beam aperture has been made gradually thin, for example, from 0.06 mm to 0.05 mm, and further, to 0.045 mm.
The step of making a portion of a metal plate thin as a material for the grid electrode of an electron gun is one of the essential sequential steps of producing the electron gun.
In general, there may be considered a method of making a portion of a metal plate thin by cutting it with a drill.
Such a method, however, has the problem that as the desired thickness of the thin plate portion to be formed at part of the metal plate becomes smaller, the relatively thinned plate portion of the metal plate may be cut off by the cutting resistance applied thereto by drilling.
For this reason, the portion of a metal plate used as a material for a grid electrode has been made thin by coining work.
Coining work is the method of making portion of a metal plate thin by coining (striking) it.
FIG. 1 is a conceptual view illustrating the coining work for a metal plate.
First, a prepared hole 51 having a diameter of xcfx86D1 is formed in a metal plate 50 as a material for a grid electrode.
A portion of the metal plate 50 around the prepared hole 51 is coined, to form the thin plate portion 52.
At this time, the coined-wall portion of the metal plate 50 runs off inwardly and outwardly.
After the coining work, a remaining hole 53 having a diameter of xcfx86D2 is thus formed by the inward run-off of the coined-wall portion.
A bulged portion 54 is also formed around the thin plate portion 52 by the outward run-off of the coined-wall portion.
FIG. 2 is a sectional view showing an essential portion of a related art first grid electrode produced by the coining work.
The essential portion of the first grid electrode G1 shown in FIG. 2, which is located around the beam aperture 60, is made gradually thin by subjecting a portion of the metal plate 61 to the coining work, which is repeated multiple times.
To make the thickness of a portion of the metal plate 61 around the beam aperture 60 as thin as material of the first grid electrode G1 from an original thickness T0 to a desired thickness t0, it is required to make the thickness of a portion outside the above portion around the beam aperture 60, into a thickness t0 xe2x80x2 by the coining work.
To repeat the coining work multiple times, the coined wall portion of the metal plate 61 must run off inwardly and outwardly for each coining work.
Accordingly, after completion of the repeated coining works, circular ribs 63 and 64 are formed around a thin plate portion 62 having the desired thickness t0.
As described above, according to the related art method, since it is required to repeat the coining work multiple times for making a portion, of a metal plate around the beam aperture thin, the circular ribs 63 and 64 are formed around the thin plate portion 62.
The presence of the circular ribs 63 and 64 around the thin plate portion 62 correspondingly requires an excess space to accommodate the ribs 63 and 64 around the thin plate portion 62.
On the other hand, for an inline type electron gun, three cathodes corresponding to three colors, red (R), green (G), and blue (B) must be provided in an inline array.
The arrangement pitch of the cathodes must be set in a specific range, typically, from 4.5 mm to 6.6 mm.
As a result, if it is required to ensure an excess space to accommodate the circular ribs 63 and 64 around the thin plate portion 62, it becomes difficult to set the arrangement pitch of the cathodes within the specific range.
To reduce the distance between the cathode and the first grid electrode, it is effective to enlarge the worked area S of the thin plate portion 62 and to set an end portion of the cathode on the worked area S.
In the case of enlarging the worked area S of the thin plate portion 62, however, the diameters of the above-described circular ribs 63 and 64 are correspondingly enlarged, with the result that the arrangement pitch of the cathodes cannot be set in the specific range.
Since the outer portion of the thin plate portion 62 is made thin into the thickness t0 xe2x80x2, it is difficult to ensure the part strength of the grid electrode required for assembly of the electron gun.
Further, as shown in FIG. 1, for subjecting the metal plate 50 to coining work, the prepared hole 51 must be provided previously in the metal plate 50.
The formation of the prepared hole 51 causes the following problem: namely, even if the diameter of the prepared hole 51 is strictly controlled, variations in the diameter and the position of the remaining hole 53 produced by the coining work occur depending on the non-controllable degree of run-off of the coined wall portion.
Accordingly, after the coining work, a beam aperture having a specific diameter must be formed in such a manner as to satisfy the condition of permitting variations in diameter and position of the remaining hole 53; that is, a condition capable of perfectly removing the remaining hole 53.
Further, as the beam aperture becomes smaller, the occupied rate of the remaining hole 53 to the beam aperture becomes larger and, therefore, in the worst case, the remaining hole 53 may remain partly upon formation of the beam aperture.
An object of the present invention is to provide an electron gun, including a grid electrode formed without any circular rib and any remaining hole by, coining and a method of producing the electron gun.
To achieve the above object, according to a first aspect of the present invention, there is provided an electron gun including a grid electrode having a thin plate portion in which an electron beam aperture is formed, wherein said thin plate portion is formed by bulging a portion of a metal plate in the plate thickness direction to such an extent as to correspond to the desired dimension of said thin plate portion, forming the bulged portion, and cutting said bulged portion.
With this configuration, it is possible to form a thin plate portion having a high dimensional accuracy without any remaining hole due to the formation of a prepared hole and any circular rib.
According to a second aspect of the present invention, there is provided a method of producing an electron gun having a thin plate portion, including the steps of bulging a portion of a metal plate as a material for a grid electrode in the plate thickness direction to such an extent as to correspond to the desired thickness of the thin plate portion, forming the bulged portion, and cutting the bulged portion, preferably, to a depth lower than the surface of the metal plate, and thereby forming the thin plate portion at part of the metal plate.
With this configuration, it is possible to form a thin plate portion having a high dimensional accuracy without any remaining hole due to the formation of a prepared hole and any circular rib.
As a result, an electron beam aperture having a desired diameter can be formed at an arbitrary position of the thin plate portion.
Further, since the thickness of a portion, of the metal plate around the thin plate portion can be kept as the original thickness of the metal plate, the worked area of the thin plate portion can be enlarged without the lack of mechanical strength required for a grid electrode for an electron gun.